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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Materials, structures, and devices for high-speed electronics

Advances in materials, devices, and instrumentation made under this grant began with ex-situ null ellipsometric measurements of simple dielectric films on bulk substrates. Today highly automated and rapid spectroscopic ellipsometers are used for ex-situ characterization of very complex multilayer epitaxial structures. Even more impressive is the in-situ capability, not only for characterization but also for the actual control of the growth and etching of epitaxial layers. Spectroscopic ellipsometry has expanded from the research lab to become an integral part of the production of materials and structures for state of the art high speed devices. Along the way, it has contributed much to our understanding of the growth characteristics and material properties. The following areas of research are summarized: Si3N4 on GaAs, null ellipsometry; diamondlike carbon films; variable angle spectroscopic ellipsometry (VASE) development; GaAs-AlGaAs heterostructures; Ta-Cu diffusion barrier films on GaAs; GaAs-AlGaAs superlattices and multiple quantum wells; superconductivity; in situ elevated temperature measurements of III-V's; optical constants of thermodynamically stable InGaAs; doping dependence of optical constants of GaAs; in situ ellipsometric studies of III-V epitaxial growth; photothermal spectroscopy; microellipsometry; and Si passivation and Si/SiGe strained-layer superlattices.

Woollam, John A.↗
NASA NTRS1995

InGaAs/InP solar cells for space application

The effects of irradiation of In(0.53)Ga(0.47)As/InP (InGaAs/InP) solar cells illuminated through a transparent InP substrate with 1 MeV electrons were measured. These solar cells were developed for bottom cells in tandem solar photovoltaic cell structures. Some InGaAs/InP heterostructures with four layers were grown by liquid phase epitaxy. The structure of the solar cells allowed lightly doped materials in n and p photoactive layers to be used. The base dopant levels ranged from 1.10(exp 17) to 5.10(exp 17) cm(exp -3). The open circuit voltage and the short circuit current were moderately degraded after irradiation with 10(exp 16) cm(exp-2) 1 MeV electrons. This behavior is explained in terms of the device structure and the n and p layer thicknesses.

Karlina, L. B.↗
NASA NTRS1995

Mechanically stacked concentrator tandem solar cells

Four-terminal mechanically stacked solar cells were developed for advanced space arrays with line-focus reflective concentrators. The top cells are based on AlGaAs/GaAs multilayer heterostructures prepared by low temperature liquid phase epitaxy. The bottom cells are based on heteroepitaxial InP/InGaAs liquid phase epitaxy or on homo-junction GaSb, Zn-diffused structures. The sum of the highest reached efficiencies of the top and bottom cells is 29.4 percent. The best four-terminal tandems have an efficiency of 27 to 28 percent. Solar cells were irradiated with 1 MeV electrons and their performances were determined as a function of fluence up to 10(exp 16) cm(exp-2). It was shown that the radiation resistance of developed tandem cells is similar to the most radiative stable AlGaAs/GaAs cells with a thin p-GaAs photoactive layer.

Andreev, V. M.↗
NASA NTRS1995

High efficient radiation stable AlGaAs/GaAs solar cells with internal Bragg reflector

An investigation of solar cells based on AlGaAs/GaAs heterostructures with an internal Bragg reflector as the back-surface reflector is presented. The Bragg reflector is grown by low pressure metalorganic chemical vapor deposition on n-GaAs substrates in a horizontal resistively heated reactor. The Bragg reflector with its maximum reflectance centered at a wavelength of 860 nm consists of 12 pairs of AlAs/GaAs layers. The resulting Bragg reflector has a thickness of 0.072 micrometers for AlAs and 0.059 micrometers for GaAs. The multi-layered quasi-dielectric stack selectively reflects weakly absorbed photons with energies near to the GaAs band gap for a second pass through the photoactive region, thus increasing the photocurrent. The use of the Bragg reflector allows the external quantum efficiency to be increased in the long wavelength of the spectrum. The use of the Bragg reflector and an antireflective coating and prismatic cover allowed an efficiency of 23.4 percent to be obtained.

Andreev, V. M.↗
NASA NTRS1997

Ka-Band GaAs FET Monolithic Power Amplifier Development

Over the course of this program, very extensive progress was made in Ka-band GaAs technology. At the beginning of the program, odd-shaped VPE MESFET wafers were used. A breakthrough in power and efficiency was achieved with highly doped (8 x 10(exp 17) cm(exp -3) MBE grown MESFET material. We obtained power of 112 mW with 16 dB gain and 21.6% efficiency at 34 GHz with a monolithic 50-100-250 micron amplifier. The next breakthrough came with the use of heterostructures grown by MBE (AlGaAs/InGaAs where the InGaAs is highly doped). This allowed us to achieve high power density with high efficiency. A benchmark 40% efficiency was achieved with a single-stage 100 micron MMIC at 32.5 GHz. The corresponding three-stage 50-100-250 micron amplifier achieved 180 mW with 23 dB gain and 30.3% efficiency. The next breakthrough came with 3-inch MBE grown PHEMT wafers incorporating an etch-stop layer for the gate recess (using RIE). Again, state-of-the-art performances were achieved: 40% efficiency with 235 mW output power and 20.7 dB gain. The single-stage 2 x 600 micron chip demonstrated 794 mW output power with 5 dB gain and 38.2% power-added efficiency (PAE). The Ka-band technology developed under this program has promise for extensive use: JPL demonstrated 32 GHz phased arrays with a three-stage amplifier developed under this contract. A variation of the three-stage amplifier was used successfully in a 4 x 4 phased array transmitter developed under another NASA contract.

Saunier, Paul↗
NASA NTRS2000

"Genetically Engineered" Nanoelectronics

The quantum mechanical functionality of nanoelectronic devices such as resonant tunneling diodes (RTDs), quantum well infrared-photodetectors (QWIPs), quantum well lasers, and heterostructure field effect transistors (HFETs) is enabled by material variations on an atomic scale. The design and optimization of such devices requires a fundamental understanding of electron transport in such dimensions. The Nanoelectronic Modeling Tool (NEMO) is a general-purpose quantum device design and analysis tool based on a fundamental non-equilibrium electron transport theory. NEW was combined with a parallelized genetic algorithm package (PGAPACK) to evolve structural and material parameters to match a desired set of experimental data. A numerical experiment that evolves structural variations such as layer widths and doping concentrations is performed to analyze an experimental current voltage characteristic. The genetic algorithm is found to drive the NEMO simulation parameters close to the experimentally prescribed layer thicknesses and doping profiles. With such a quantitative agreement between theory and experiment design synthesis can be performed.

Klimeck, Gerhard↗
NASA NTRS2000

Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport

Interest in optical devices which can operate in the visible spectrum has motivated research interest in the II-VI wide band gap semiconductor materials. The recent challenge for semiconductor opto-electronics is the development of a laser which can operate at short visible wavelengths. In the past several years, major advances in thin film technology such as molecular beam epitaxy and metal organic chemical vapor deposition have demonstrated the applicability of II-VI materials to important devices such as light-emitting diodes, lasers, and ultraviolet detectors. With an energy gap of 2.7 eV at room temperature, and an efficient band- to-band transition, ZnSe has been studied extensively as the primary candidate for a blue light emitting diode for optical displays, high density recording, and military communications. By employing a ternary or quaternary system, the energy band gap of II-VI materials can be tuned to a specific range. While issues related to the compositional inhomogeneity and defect incorporation are still to be fully resolved, ZnSe bulk crystals and ZnSe-based heterostructures such as ZnSe/ZnSeS, ZnSe/ZnCdSe and ZnCdSe/ZnSeS have showed photopumped lasing capability in the blue-green region at a low threshold power and high temperatures. The demonstration of its optical bistable properties in bulk and thin film forms also make ZnSe a possible candidate material for the building blocks of a digital optical computer. Despite this, developments in the crystal growth of bulk H-VI semiconductor materials has not advanced far enough to provide the low price, high quality substrates needed for the thin film growth technology.

Su, Ching-Hua↗
NASA NTRS2000

Growth and Characterization of III-V Semiconductors for Device Applications

The research goal was to achieve a fundamental understanding of the physical processes occurring at the surfaces and interfaces of epitaxially grown InGaAs/GaAs (100) heterostructures. This will facilitate the development of quantum well devices for infrared optical applications and provide quantitative descriptions of key phenomena which impact their performance. Devices impacted include high-speed laser diodes and modulators for fiber optic communications at 1.55 micron wavelengths and intersub-band lasers for longer infrared wavelengths. The phenomenon of interest studied was the migration of indium in InGaAs structures. This work centered on the molecular beam epitaxy reactor and characterization apparatus donated to CAU by AT&T Bell Laboratories. The material characterization tool employed was secondary ion mass spectrometry. The training of graduate and undergraduate students was an integral part of this program. The graduate students received a thorough exposure to state-of-the-art techniques and equipment for semiconductor materials analysis as part of the Master''s degree requirement in physics. The undergraduates were exposed to a minority scientist who has an excellent track record in this area. They also had the opportunity to explore surface physics as a career option. The results of the scientific work was published in a refereed journal and several talks were presented professional conferences and academic seminars.

Williams, Michael D.↗
NASA NTRS2001

A Radiation-Tolerant, Low-Power Non-Volatile Memory Based on Silicon Nanocrystal Quantum Dots

Nanocrystal nonvolatile floating-gate memories are a good candidate for space applications - initial results suggest they are fast, more reliable and consume less power than conventional floating gate memories. In the nanocrystal based NVM device, charge is not stored on a continuous polysilicon layer (so-called floating gate), but instead on a layer of discrete nanocrystals. Charge injection and storage in dense arrays of silicon nanocrystals in SiO2 is a critical aspect of the performance of potential nanocrystal flash memory structures. The ultimate goal for this class of devices is few- or single-electron storage in a small number of nanocrystal elements. In addition, the nanocrystal layer fabrication technique should be simple, 8-inch wafer compatible and well controlled in program/erase threshold voltage swing was seen during 100,000 program and erase cycles. Additional near-term goals for this project include extensive testing for radiation hardness and the development of artificial layered tunnel barrier heterostructures which have the potential for large speed enhancements for read/write of nanocrystal memory elements, compared with conventional flash devices. Additional information is contained in the original extended abstract.

Bell, L. D.↗
NASA NTRS2001

Long-Term Reliability of SiGe/Si HBTs From Accelerated Lifetime Testing

Accelerated lifetime tests were performed on double-mesa structure Si(0.7)Ge(0.3)/Si npn heterojunction bipolar transistors, grown by molecular beam epitaxy, in the temperature range of 175 C-275 C. The transistors (with 5x20 sq micron emitter area) have DC current gains approx. 40-50 and f(sub T) and f(sub max) of up to 22 GHz and 25 GHz, respectively. It is found that a gradual degradation in these devices is caused by the recombination enhanced impurity diffusion (REID) of boron atoms from the p-type base region and the associated formation of parasitic energy barriers to electron transport from the emitter to collector layers. This REED has been quantitatively modeled and explained, to the first order of approximation, and the agreement with the measured data is good. The mean time to failure (MTTF) of these devices at room temperature under 1.35 x 10(exp 4) A/sq cm current density operation is estimated from the extrapolation of the Arrhenius plots of device lifetime versus reciprocal temperature. The results of the reliability tests offer valuable feedback for SiGe heterostructure design in order to improve the long-term reliability of the devices and circuits made with them. Hot electron induced degradation of the base-emitter junction was also observed during the accelerated lifetime testing. In order to improve the HBT reliability endangered by the hot electrons, deuterium sintered techniques have been proposed. The preliminary results from this study show that a deuterium-sintered HBT is, indeed, more resistant to hot-electron induced base-emitter junction degradation.

Bhattacharya, Pallab↗
NASA NTRS2003

Evolution of Local Microstructures: Spatial and Temporal Correlation in Clusters Undergoing 2-Dimensional Diffusion

Diffusion-limited, capillarity-driven, coarsening of precipitates is an important and intensively studied phenomenon. Coarsening can occur among several microstructural constituents ranging from the primary phases to widely dispersed precipitates. Moreover, coarsening of three-dimensional (3-D) phase domains, occurring via mass transport through a two-dimensional (2-D) diffusion field (called mixed-dimensional phase coarsening) is a cooperative phenomenon that is of practical importance, especially in thin film technologies and catalysis. A primary objective of our current work is to focus on the details of evolving microstructures through a study of 'island' formation in heteroepitaxial film/substrate systems. Whereas this work has been designed for space-based coarsening studies, which require long-duration quality microgravity to observe 3-D Ostwald ripening, a significant amount of work has been performed during ground-based studies to observe diffusional coarsening in a liquid-liquid two-phase system. We have performed the experiment at an isopycnic point to maximize exclusion of gravity-based system disturbance. This has allowed observations of 3-D droplets, immobilized by container wall contact, undergoing coarsening by 2-D diffusion for the required long times (approximately 4 months) using a holographic imaging technique. Although such systems may not be as reliant on long-duration microgravity processing as 3-D systems are, they are of direct relevance to so-called mixed-dimensional coarsening and to the study of heterostructures grown by thin-film technique

Curreri, Peter A.↗
NASA NTRS2003

Real-Time Optical Monitoring and Simulations of Gas Phase Kinetics in InN Vapor Phase Epitaxy at High Pressure

Understanding the kinetics of nucleation and coalescence of heteroepitaxial thin films is a crucial step in controlling a chemical vapor deposition process, since it defines the perfection of the heteroepitaxial film both in terms of extended defect formation and chemical integrity of the interface. The initial nucleation process also defines the film quality during the later stages of film growth. The growth of emerging new materials heterostructures such as InN or In-rich Ga(x)In(1-x)N require deposition methods operating at higher vapor densities due to the high thermal decomposition pressure in these materials. High nitrogen pressure has been demonstrated to suppress thermal decomposition of InN, but has not been applied yet in chemical vapor deposition or etching experiments. Because of the difficulty with maintaining stochiometry at elevated temperature, current knowledge regarding thermodynamic data for InN, e.g., its melting point, temperature-dependent heat capacity, heat and entropy of formation are known with far less accuracy than for InP, InAs and InSb. Also, no information exists regarding the partial pressures of nitrogen and phosphorus along the liquidus surfaces of mixed-anion alloys of InN, of which the InN(x)P(1-x) system is the most interesting option. A miscibility gap is expected for InN(x)P(1-x) pseudobinary solidus compositions, but its extent is not established at this point by experimental studies under near equilibrium conditions. The extension of chemical vapor deposition to elevated pressure is also necessary for retaining stoichiometric single phase surface composition for materials that are characterized by large thermal decomposition pressures at optimum processing temperatures.

Dietz, Nikolaus↗
NASA NTRS2003

Evolution of Local Microstructures of Clusters Undergoing 2-Dimensional Diffusion

A primary objective of our current work is to study the details of evolving microstructures through a study of "island" formation in heteroepitaxial film/substrate systems. Such systems could be of direct relevance to so-called mixed-dimensional coarsening and to the study of heterostructures grown by thin-film techniques. The spontaneous formation of three-dimensional islands in strained-layer heteroepitaxy, e.g., has emerged as a new technique for the synthesis of self-assembled quantum dots. This work focuses on coarsening by two-dimensional diffusion of three-dimensional droplets on a flat surface.

Frazier, Donald O.↗
NASA NTRS2003

Long-Term Reliability of High Speed SiGe/Si Heterojunction Bipolar Transistors

Accelerated lifetime tests were performed on double-mesa structure Si/Si0.7Ge0.3/Si npn heterojunction bipolar transistors, grown by molecular beam epitaxy, in the temperature range of 175C-275C. Both single- and multiple finger transistors were tested. The single-finger transistors (with 5x20 micron sq m emitter area) have DC current gains approximately 40-50 and f(sub T) and f(sub MAX) of up to 22 GHz and 25 GHz, respectively. The multiple finger transistors (1.4 micron finger width, 9 emitter fingers with total emitter area of 403 micron sq m) have similar DC current gain but f(sub T) of 50 GHz. It is found that a gradual degradation in these devices is caused by the recombination enhanced impurity diffusion (REID) of boron atoms from the p-type base region and the associated formation of parasitic energy barriers to electron transport from the emitter to collector layers. This REID has been quantitatively modeled and explained, to the first order of approximation, and the agreement with the measured data is good. The mean time to failure (MTTF) of the devices at room temperature is estimated from the extrapolation of the Arrhenius plots of device lifetime versus reciprocal temperature. The results of the reliability tests offer valuable feedback for SiGe heterostructure design in order to improve the long-term reliability of the devices and circuits made with them. Hot electron induced degradation of the base-emitter junction was also observed during the accelerated lifetime testing. In order to improve the HBT reliability endangered by the hot electrons, deuterium sintered techniques have been proposed. The preliminary results from this study show that a deuterium-sintered HBT is, indeed, more resistant to hot-electron induced base-emitter junction degradation. SiGe/Si based amplifier circuits were also subjected to lifetime testing and we extrapolate MTTF is approximately 1.1_10(exp 6) hours at 125iC junction temperature from the circuit lifetime data.

Ponchak, George E.↗
NASA NTRS2004

Microscopic Theory and Simulation of Quantum-Well Intersubband Absorption

We study the linear intersubband absorption spectra of a 15 nm InAs quantum well using the intersubband semiconductor Bloch equations with a three-subband model and a constant dephasing rate. We demonstrate the evolution of intersubband absorption spectral line shape as a function of temperature and electron density. Through a detailed examination of various contributions, such as the phase space filling effects, the Coulomb many-body effects and the non-parabolicity effect, we illuminate the underlying physics that shapes the spectra. Keywords: Intersubband transition, linear absorption, semiconductor heterostructure, InAs quantum well

Li, Jianzhong↗
NASA NTRS2004

Novel Biomedical Devices Utilizing Light-Emitting Nanostructures

As part of the NASA project, we are investigating the formation, properties, and performance of QD heterostructures, to be incorporated into a novel biomedical device for detecting bacteria and/or viruses in fluids on board space vehicles. We are presently synthesizing the epitaxial quantum dot structures using molecular beam epitaxy. We recently developed a method for controlling the arrangement of QDs, based upon a combination of buffer layer growth and controlled annealing sequences. This method is promising for producing arrangements of QDs with a locally well-controlled distribution of sizes. In the future, we plan to explore selective pre-patterning of the starting surface using focused ion-beam nanopatterning, which will enable us to precisely tune the compositions, sizes, and placement of the QDs, in order laterally tune the emission and detection wavelengths of QD based devices.

Goldman, Rachel S.↗
NASA NTRS2004

Passivating Window/First Layer AR Coating for Space Solar Cells

Chemically grown oxides, if well designed, offer excellent surface passivation of the emitter surface of space solar cells and can be used as effective passivating window/first layer AR coating. In this paper, we demonstrate the effectiveness of using a simple room temperature wet chemical technique to grow cost effective passivating layers on solar cell front surfaces after the front grid metallization step. These passivating layers can be grown both on planar and porous surfaces. Our results show that these oxide layers: (i) can effectively passivate the from the surface, (ii) can serve as an effective optical window/first layer AR coating, (iii) are chemically, thermally and UV stable, and (iv) have the potential of improving the BOL and especially the EOL efficiency of space solar cells. The potential of using this concept to simplify the III-V based space cell heterostructures while increasing their BOL and EOL efficiency is also discussed.

Faur, Mircea↗